1. Field of the Invention
This invention resides in the field of the desulfurization of petroleum and petroleum-based fuels.
2. Description of the Prior Art
While alternative sources of power are under development and in use in many parts of the world, fossil fuels remain the largest and most widely used source due to their high efficiency, proven performance, and relatively low prices. Fossil fuels take many forms, ranging from petroleum fractions to coal, tar sands, and shale oil, and their uses extend from consumer uses such as automotive engines and home heating to commercial uses such as boilers, furnaces, smelting units, and power plants.
A persistent problem in the processing and use of fossil fuels is the presence of sulfur, notably in the form of organic sulfur compounds. Sulfur has been implicated in the corrosion of pipeline, pumping, and refining equipment and in the premature failure of combustion engines. Sulfur is also responsible for the poisoning of catalysts used in the refining and combustion of fossil fuels. By poisoning the catalytic converters in automotive engines, sulfur is responsible in part for the emissions of oxides of nitrogen (NOx) from diesel-powered trucks and buses. Sulfur is also responsible for the particulate (soot) emissions from trucks and buses since the traps used on these vehicles for controlling these emissions are quickly degraded by high-sulfur fuels. Perhaps the most notorious characteristic of sulfur compounds in fossil fuels is the conversion of the sulfur in these compounds to sulfur dioxide when the fuels are combusted. The release of sulfur dioxide to the atmosphere results in acid rain, a deposition of acid that is harmful to agriculture, wildlife, and human health. Ecosystems of various kinds are threatened with irreversible damage, as is the quality of life.
In response to these concerns, the Clean Air Act of 1964 was enacted, and various amendments, including those of 1990 and 1999, have imposed progressively more stringent requirements to reduce even further the amount of sulfur released to the atmosphere. In a recent action, the United States Environmental Protection Agency has lowered the sulfur standard for diesel fuel to 15 parts per million by weight (ppmw), effective in mid-2006, from the present standard of 500 ppmw. For reformulated gasoline, the current standard of 300 ppmw has been lowered to 30 ppmw, effective Jan. 1, 2004. Similar changes have been enacted in the European Union, which will enforce a limit of 50 ppmw on the sulfur limit for both gasoline and diesel fuel in the year 2005.
Because of these regulatory actions, the need for more effective desulfurization methods is always present. In addition to the difficulty in lowering sulfur emissions to meet the requirements, the petroleum industry also faces the increased production costs associated with sophisticated desulfurization methods and the unfavorable reactions of consumers and governments to increased prices. The costs associated with fossil fuels are some of the major factors affecting the world economy.
The most common method of desulfurization of fossil fuels is hydrodesulfurization, in which the fossil fuel is reacted with hydrogen gas at elevated temperature and high pressure in the presence of a costly catalyst. Organic sulfur is reduced by this reaction to gaseous H2S, which is then oxidized to elemental sulfur by the Claus process. Unreacted H2S from the process is harmful, however, even in very small amounts. H2S has an extremely high acute toxicity, which has caused many deaths in the workplace and in areas of natural accumulation, and is hazardous to workers. These hazards present health risks in many types of industries, such as the gas, oil, chemical, geothermal energy, mining, drilling, and smelting industries. Even brief exposure to H2S at a concentration of 140 mg/m3 causes conjunctivitis and keratitis, while exposures at 280 mg/m3 and above can cause loss of consciousness, paralysis, and even death. H2S exposure has been implicated in disorders of the nervous system, and in cardiovascular, gastrointestinal, and ocular disorders. One of the difficulties with the new regulations is that when hydrodesulfurization is performed under the more stringent conditions needed to achieve the lower sulfur levels, there is an increased risk of hydrogen leaking through walls of the reactor.
In addition to its tendency to release H2S into the atmosphere, the hydrodesulfurization process has certain limitations in its ability to convert the variety of organic sulfur compounds that are present in fossil fuels. Among these compounds, mercaptans, thioethers, and disulfides are relatively easy to remove by the process. Other sulfur-bearing organic compounds however are less easy to remove and require harsher reaction conditions. These compounds include aromatic compounds, cyclic compounds, and condensed multicyclic compounds. Illustrative of these compounds are thiophene, benzothiophene, dibenzothiophene, other condensed-ring thiophenes, and various substituted analogs of these compounds. These compounds, which account for upwards of 40% of the total sulfur content of crude oils from the Middle East and upwards of 70% of the sulfur content of West Texas crude oil, are the most difficult to remove, and for this reason is commonly the focus of desulfurization studies. The reaction conditions needed to remove these compounds are so harsh that they cause degradation of the fuel itself, thereby lowering its quality.
It has now been discovered that organic sulfur compounds can be removed from a fossil (or petroleum-derived) fuel by a process that combines oxidative desulfurization with the use of ultrasound. The oxidative desulfurization is achieved by combining the fossil fuel with a hydroperoxide oxidizing agent in the presence of an aqueous fluid, and the ultrasound is applied to the resulting mixture to increase the reactivity of the species in the mixture. An indication of the unusually high effectiveness of the process is the observation that dibenzothiophene and related sulfur-bearing organic sulfides, which are the most refractory organic sulfur compounds in fossil fuels, are readily converted by this process to the corresponding sulfones under relatively modest conditions of temperature and pressure. The higher polarities of the sulfones relative to the sulfides render the sulfones readily susceptible to removal by conventional polarity-based separation processes. Thus, dibenzothiophenes and other sulfides of comparable or lesser resistance to oxidation are convertible by this process to their more polar sulfone analogs, without externally applied heat or pressure other than that which may be caused internally in a highly localized manner by the ultrasound.
An advantage of the process of this invention is that the oxidation is selective toward the conversion of sulfur-bearing compounds and occurs with no apparent change in the non-sulfur-bearing components of the fossil fuel. In addition, although both aqueous and organic phases remain in an emulsion form present throughout the progress of the reaction, the process can be performed to useful effect without the addition of a surface active agent. While not intending to be bound by any particular theory, it is believed that most fossil fuels contain native (i.e., naturally present) components that serve as surfactants. A still further advantage is that the conversion occurs in a very short period of time, i.e., considerably less than an hour, preferably less than twenty minutes, and in many cases less than ten minutes.